CM3DS: fix a minor bug in the ethernet LWIP driver

This patch fixes a memory bug. `eth_arch_enetif_init` method call
would attempt to write to un-initialized area of memory.

Change-Id: I9881de71d58fa14db609fe3e24617a210b896471
Signed-off-by: Galanakis, Minos <[email protected]>

Author: minosgalanakis

features/FEATURE_LWIP/lwip-interface/lwip-eth/arch/TARGET_CM3DS_MPS2/mps2_emac.c

@@ -1,5 +1,5 @@
 /* mbed Microcontroller Library
- * Copyright (c) 2017 ARM Limited
+ * Copyright (c) 2017-2018 ARM Limited
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -316,16 +316,16 @@ err_t eth_arch_enetif_init(struct netif *netif)
     err_t error = ERR_OK;
     struct ethernetif *ethernetif;
 
-    ethernetif->is_enabled = 0;
-
     LWIP_ASSERT("netif != NULL", (netif != NULL));
-
     ethernetif = mem_malloc(sizeof(struct ethernetif));
+
     if (ethernetif == NULL) {
         LWIP_DEBUGF(NETIF_DEBUG, ("ethernetif_init: out of memory\n"));
         return ERR_MEM;
     }
 
+    ethernetif->is_enabled = 0;
+
 #if LWIP_NETIF_HOSTNAME
     /* Initialize interface hostname */
     netif->hostname = HOSTNAME_STRING;